Use of LED for illumination

Pao Yue-kong Library Electronic Theses Database

Use of LED for illumination


Author: Chan, Ka-wai
Title: Use of LED for illumination
Degree: M.Eng.
Year: 2010
Subject: Hong Kong Polytechnic University -- Dissertations
Light emitting diodes -- Evaluation
Light emitting diodes -- Energy consumption
Department: Dept. of Building Services Engineering
Pages: 99 leaves : ill. ; 31 cm.
InnoPac Record:
Abstract: A set of laboratory experiments were conducted to investigate the performance characteristics of high power LEDs under various cooling device, current and voltage. Light outputs properties are as a function of time were measured for three types of LEDs. In the first stage. LED arrays were tested under normal drive current and voltage, and higher drive current with various configurations: (1) installed with heat sink only; (2) installed with cooling fan and heat sink. At the second stage, the LED arrays are operated for 700 hours to investigate the degradation of the lighting output and the energy consumption with the mentioned configurations. According to the stage 1 result, the LED arrays in configuration 1 and 2 have the similar light output and electrical consumption at the rated current. In a higher driving current, the LED array for configuration 1 had a decrease of the light output and the heat sink temperature was raised rapidly, but configuration 2 had a lower rate for the temperature rise and a higher light output than configuration 1. Due to the absence of the cooling device, more electrical energy was converted to the heat toss instead of the visible light, the luminous output was decreased. The heating of the LED induces internal and external thermomechanical stress due to the Coefficient of Thermal Expansion mismatch. The decrease of the luminous output was caused by the decrease of the internal quantum efficiency. Due to the decrease of the luminous output, the efficiency was decreased.
In addition, the optimization of the LED was stimulated under various conditions. By choosing the appropriate setting, it is based on three major elements which are the luminous efficiency, temperature and forward current. The rating of the cooling device should also be considered, because the rating will influence the performance of the LED's heat rejection and total electrical consumption of the whole lighting device. At the stage 2, two set up with various configuration were operated for one month, the result showed that the degradation of the light output for configuration 1 is faster than configuration 2, and also the electrical consumption for configuration 2 was much lesser that the configuration 1. The usage for the cooling device can enable a better performance for the LED array. The selection for the cooling device depends on the electrical consumption and heat rejection performance and the electrical consumption should be minimized to reduce the electrical consumption for the LEDs. Low energy consumption is the major reason for replacing the traditional lighting product by SSL product.

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